In recent years, from the aspects of effective use of resources, prevention of environmental pollution, and the like, solar cells for converting sunlight to electric energy have been used extensively, and further developed progressively.
Crystalline silicon solar cells are generally configured, as shown in FIG. 1, to have a plurality of photovoltaic cells 14 such as silicon power generation elements sealed with a front encapsulant 13A and a back encapsulant 13B such as ethylene-vinyl acetate polymers (EVA) films interposed between a front transparent protective member 11 and a back protective member (back cover) 12 composed of glass substrates or the like, and reinforced with a frame 15.
Thin-film silicon, cadmium-tellurium solar cells are generally configured, as shown in FIG. 2, to have a photovoltaic cell 16 prepared on a front transparent protective member 11 composed of a glass substrate or the like and have a back protective member (back cover) 12, which are sealed with an encapsulant 13 such as plasticized polyvinyl butyral and ethylene-vinyl acetate polymers (EVA) films interposed therebetween, and reinforced with a frame 15.
CIGS and CIS solar cells are generally configured, as shown in FIG. 3, to have a photovoltaic cell 16 prepared on a back protective member 12 composed of a glass substrate or the like and have a front transparent protective member 11 composed of glass, which are sealed with an encapsulant 13 such as plasticized polyvinyl butyral and ethylene-vinyl acetate polymers (EVA) films interposed therebetween, and reinforced with a frame 15.
While conventional solar cells are generally implemented through reinforcement with a frame as described above, further improvements in durability and reductions in cost have been required for the solar cells. The price of a frame is approximately twice to four times as high as that of an encapsulant, and frameless implementation is very useful for reductions in cost for solar cells.
Therefore, frameless solar cells have been proposed (for example, THE SOLAR STANDARD, No. 1). However, it is often the case that the back sides require a reinforcing structure in order to pass load bearing tests, and when the back sides require no reinforcing structure, expensive thermal toughening glass is used, and there has been thus a demand for further reductions in cost. While plasticized polyvinyl butyral is used extensively as the encapsulant in this case, property degradation is observed around a high temperature (50° C.), and there has been also a demand for improvements from the standpoint of durability. Due to the fact that solar cells generate heat during power generation, there is actually a possibility of a temperature rise to 50° C. or higher even in a usage environment at 20° C., and the load bearing property at high temperatures is thus of great importance.
In addition, the shock resistance is also of great importance in order to pass the MST32 shock fracture test in accordance with the Photovoltaic (PV) Module Safety Qualification (IEC61730-2).
In this context, it has been known that a polyvinyl butyral resin with a high volume resistivity is used as an encapsulant for solar cells (for example, WO 2009/151952A). However, in an example, the amount of a plasticizer used is large, with the result that properties are inadequate around 50° C., and the fluidity at a lamination temperature is inadequate, with the result that defects in appearance are likely to be caused, such as bubbles and defective adhesion.
Now, polyvinyl butyral resins have been conventionally formed into sheets, and used extensively as interlayer films for laminated glass such as window panes of buildings. However, glass plates may deviate by, or cause bubbles to be generated by heat in some cases after attaching the plates to each other, and furthermore, there is the problem of inadequate load bearing property, such as glass broken easily. In this context, sheets composed of ionomer resins have been proposed as glass laminates for danger-resistance window facilities, which are excellent in load bearing property (for example, JP 2002-514533A). However, the sheets of ionomer resins have low heat resistance, and inadequate properties around 50° C.